The present invention relates to balanced processing based on selection of a receiver, and more particularly to delay spread estimation for a radio frequency signal path, selection of a receiver equalizer based on such estimation, and balanced processing based on such selection. Even more particularly, the present invention relates to delay spread estimation of a cellular telephone signal path, selection of a receiver equalizer based on such estimation, wherein a delay spread threshold for selection of the receiver equalizer is a function of signal-to-noise ratio, and balanced voice processing based on such receiver selection.
Communication channels in the cellular environment commonly impose a combination of distorting effects on transmitted signals. Rayleigh fading, where a signal's perceived power level rises and falls rapidly over a wide range, results from the combination (interference) of signals that have traversed paths differing in length by at least a significant fraction of a wavelength (i.e., about 30 cm. for cellular). This type of interference is known as multi-path interference. Differences in path transmission times that approach the time taken to transmit a symbol result in a second problem called delay spread.
Delay spread results in reception of multiple delayed replicas of a transmitted signal. Each Rayleigh faded replica has randomly distributed amplitude and phase, and the rate at which this complex quantity varies is constrained by the Doppler bandwidth associated with a vehicle's speed. In a frequency nonselective environment, the sampled outputs of a receiver's matched filter provide uncorrelated estimates of the transmitted data. As such, in terms of discrete time samples, the channel has exhibited an impulse response proportional to a delta function. With delay spread, on the other hand, the discrete time channel impulse response is extended to introduce energy at a number of symbol times. The effect of the channel on the transmitted signal, in turn, may be viewed as the convolution of the transmitted information with the channel's impulse response. The channel, therefore, emulates a convolutional coding process (encoder).
This leads to the possibility of estimating the transmitted information through the use of methods analogous to typical decoding of convolutional codes, i.e., maximum likelihood sequence estimation techniques.
Such maximum likelihood sequence estimation techniques, when implemented in a cellular telephone receiver, provide improved performance in the receive path, when multiple replicas of the transmitted signals are received. However, when multiple replicas are not received, i.e., when the transmitted signal is received having traversed a single signal path, such maximum likelihood sequence estimation receivers actually degrade performance. Thus, it is desirable to switch between a receiver that performs maximum likelihood sequence estimation, and a receiver that does not perform such maximum likelihood sequence estimation.
Both of these receivers can be implemented using a digital signal processor. Thus it is desirable to load and execute either an equalizing receiver routine or a non-equalizing receiver routine in the digital signal processor depending on whether multi-path interference is detected. The equalizing receiver requires more processing from the digital signal processor.
For the North American digital cellular system, a number of documents define the standards of implemented components. With respect to this invention, the following are of interest: "Dual-Mode Mobile Station-Base Station Compatibility Standard" denoted here as IS-54, EIA/TIA Project Number 2398, Rev. A Jan 1991; "Recommended Minimum Performance Standards for 800 MHz Dual-Mode Mobile Stations", denoted here as IS-55, EIA/TIA Project Number 2216, Apr. 1991; and Recommended Minimum Performance Standards for Full Rate Speech Codes denoted here as IS-85, TIA/EIA, May 1992.
In accordance with these standards, a voice encoder/decoder is used to perform voice processing, or speech coding. Voice processing is not, however, specified by the above specifications with bit-exact description. Instead such voice processing is described in functional form, leaving the exact implementation of the voice processing to the designer. Thus, more complex voice processing techniques can be utilized to achieve higher quality voice output, and lower quality voice processing techniques can be utilized to achieve lower quality voice output, while still conforming to the above-mentioned standards. The higher quality voice processing techniques employ more complicated processing, and therefore require more powerful and more costly, or additional digital signal processors to implement. This is particularly true if the equalizing receiver, which also employs more complicated processing, is to be implemented simultaneously with such higher quality voice processing techniques.
The present invention advantageously provides for balanced processing, which allows for the use of higher quality voice processing (or other high complexity processing), and selection between equalizing and non-equalizing receivers, while minimizing the digital signal processing power needed. As a result, the invention also minimizes the cost required, for implementation.